Magnetic resonance imaging apparatus and magnetic resonance imaging method

ABSTRACT

A magnetic resonance imaging apparatus includes an acoustic control unit and an image data acquisition unit. The acoustic control unit applies a gradient magnetic field for controlling a sound in synchronization with a signal representing a respiratory body motion. The image data acquisition unit acquires imaging data by subsequently imaging to control the sound and generate image data based on the imaging data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of copending application Ser. No. 12/169,880filed Jul. 9, 2008, which claims priority based on Japanese PatentApplication No. 2007-181894 filed Jul. 11, 2007, and Japanese PatentApplication No. 2008-151575 filed Jun. 10, 2008, the entire contents ofall of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present exemplary embodiment relates to a MRI (magnetic resonanceimaging) apparatus and a magnetic resonance imaging method which excitesnuclear spins of an object magnetically with an RF (radio frequency)signal having the Larmor frequency and reconstructs an image based onNMR (nuclear magnetic resonance) signals generated due to the excitationand, more particularly, to a magnetic resonance imaging apparatus and amagnetic resonance imaging method which make it possible to suppress orreduce influence of respiratory body motion on imaging.

2. Description of Related Art

Magnetic Resonance Imaging is an imaging method which magneticallyexcites nuclear spins of an object set in a static magnetic field withan RF signal having the Larmor frequency and reconstructs an image basedon an NMR signal generated due to the excitation.

In MRI imaging of the upper abdominal area such as a liver, afluctuation of an image and an artifact called ghost where the outlineof a body surface would be thinly overlapped occur due to respiratorybody motion. To avoid these, there is the conventional technique forreducing the influence of respiratory body motion by sorting an encodingorder in accordance with a breathing period.

Further, suppression of image degradation is performed by alleviatingthe object's breathing movement by means of the auto voice function tostabilize a TR (repetition time) (see, for example, Japanese PatentApplication (Laid-Open) No. 2001-346773).

When imaging is performed while holding the breath, even if a parallelimaging that is a high-speed imaging technique for acquiring signalswith multiple surface coils is applied, an imaging time is limited to10-30 seconds. An imaging time here becomes TR×(a matrix size in a phaseencode direction). Therefore, when imaging is performed while holdingthe breath, it is difficult to improve a matrix size in a phase encodedirection, i.e., a resolution. Moreover, since imaging time is short,SNR (signal-to-noise ratio) is also limited, and imaging with a high SNRand a high resolution for examining detailed disease states isdifficult.

Meanwhile, when imaging is performed with the conventionalbreathing-compensated method, a large difference occurs in image qualityof obtained images depending on the stability of a heart rate of theobject. This means if both a breathing level of the object and abreathing period stay constant, an ideal synchronous imaging can beperformed. However, when breathing of the object becomes erratic,disturbed amplitude and phase because of breathing exist in acquireddata even if the conventional correction in a body motion is performedand, therefore, this disturbance appears as an image artifact.

SUMMARY OF EXEMPLARY EMBODIMENTS

The present exemplary embodiment has been made in light of theconventional situations, and it is an object of the present exemplaryembodiment to provide a magnetic resonance imaging apparatus and amagnetic resonance imaging method which make it possible to suppress orreduce influence of respiratory body motion on imaging.

The present exemplary embodiment provides a magnetic resonance imagingapparatus comprising: an acoustic control unit configured to apply agradient magnetic field for controlling a sound in synchronization witha signal representing a respiratory body motion; and an image dataacquisition unit configured to acquire imaging data by imagingsubsequently to control the sound and generate image data based on theimaging data.

The present exemplary embodiment provides a magnetic resonance imagingapparatus comprising: a correction data acquisition unit configured toacquire correction use data for correcting an influence of a respiratorybody motion on image data in synchronization with a signal representingthe respiratory body motion; a data acquisition unit configured toacquire imaging data by imaging subsequently to acquiring the correctionuse data; and a correction unit configured to generate image data havinga corrected influence of the respiratory body motion based on thecorrection use data from the imaging data.

The present exemplary embodiment provides a magnetic resonance imagingapparatus comprising: a correction data acquisition unit configured toacquire correction use data for correcting an influence of a respiratorybody motion on image data; an acoustic control unit configured tocontrol a sound by applying a gradient magnetic field with a purposeother than acquiring the correction use data; a data acquisition unitconfigured to acquire imaging data by imaging subsequently to acquirethe correction use data and control the sound; and a correction unitconfigured to generate image data having a corrected influence of therespiratory body motion based on the correction use data from theimaging data.

The present exemplary embodiment provides a magnetic resonance imagingmethod comprising: applying a gradient magnetic field for controlling asound in synchronization with a signal representing a respiratory bodymotion; and acquiring imaging data by imaging subsequently to controlthe sound and generate image data based on the imaging data.

The present exemplary embodiment provides a magnetic resonance imagingmethod comprising: acquiring correction use data for correcting aninfluence of a respiratory body motion on image data in synchronizationwith a signal representing the respiratory body motion; acquiringimaging data by imaging subsequently to acquire the correction use data;and generating image data having a corrected influence of therespiratory body motion based on the correction use data from theimaging data.

The magnetic resonance imaging apparatus and the magnetic resonanceimaging method as described above make it possible to suppress or reduceinfluence of respiratory body motion on imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing a magnetic resonance imaging apparatusaccording to an exemplary embodiment of the present invention;

FIG. 2 is a functional block diagram of the computer shown in FIG. 1;

FIG. 3 is a diagram showing timing for performing an acoustic controlsequence set by the imaging condition setting unit shown in FIG. 2 insynchronization with breathing;

FIG. 4 is a diagram showing an example of acoustic control sequenceshown in FIG. 3;

FIG. 5 is a diagram showing another example of acoustic control sequenceshown in FIG. 3;

FIG. 6 is a diagram showing timing for performing a correction usesequence set by the imaging condition setting unit shown in FIG. 2 insynchronization with breathing;

FIG. 7 is a diagram explaining relationship between correction use dataacquired by the correction use sequence shown in FIG. 6 and arespiratory motion amount of the object;

FIG. 8 is a diagram showing an example case of setting one dimensionalsequence as the correction use sequence shown in FIG. 6;

FIG. 9 is a diagram showing an example case of setting two-dimensionalsequence as the correction use sequence shown in FIG. 6;

FIG. 10 is a flowchart for generating an image of the object whileacoustically controlling and motion correcting regarding respiratorybody motion in synchronization with breathing using the magneticresonance imaging apparatus shown in FIG. 1;

FIG. 11 is a diagram explaining a technique for reducing artifactsreferred to as HIGH SORT which can be applied to the magnetic resonanceimaging apparatus shown in FIG. 1; and

FIG. 12 is a diagram explaining a technique for reducing artifactsreferred to as LOW SORT which can be applied to the magnetic resonanceimaging apparatus shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A magnetic resonance imaging apparatus and a magnetic resonance imagingmethod according to exemplary embodiments of the present invention willbe described with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a magnetic resonance imaging apparatusaccording to an exemplary embodiment of the present invention.

A magnetic resonance imaging apparatus 20 includes a static field magnet21 for generating a static magnetic field, a shim coil 22 arrangedinside the static field magnet 21 which is cylinder-shaped, a gradientcoil 23 and an RF coil 24. The static field magnet 21, the shim coil 22,the gradient coil 23 and the RF coil 24 are built in a gantry (notshown).

The magnetic resonance imaging apparatus 20 also includes a controlsystem 25. The control system 25 includes a static magnetic field powersupply 26, a gradient power supply 27, a shim coil power supply 28, atransmitter 29, a receiver 30, a sequence controller 31 and a computer32. The gradient power supply 27 of the control system 25 includes anX-axis gradient power supply 27 x, a Y-axis gradient power supply 27 yand a Z-axis gradient power supply 27 z. The computer 32 includes aninput device 33, a display unit 34, an operation unit 35 and a storageunit 36.

The static field magnet 21 communicates with the static magnetic fieldpower supply 26. The static magnetic field power supply 26 supplieselectric current to the static field magnet 21 to get the function togenerate a static magnetic field in an imaging region. The static fieldmagnet 21 includes a superconductivity coil in many cases. The staticfield magnet 21 gets current from the static magnetic field power supply26 which communicates with the static field magnet 21 at excitation.However, once excitation has been made, the static field magnet 21 isusually isolated from the static magnetic field power supply 26. Thestatic field magnet 21 may include a permanent magnet which makes thestatic magnetic field power supply 26 unnecessary.

The static field magnet 21 has the cylinder-shaped shim coil 22coaxially inside itself. The shim coil 22 communicates with the shimcoil power supply 28. The shim coil power supply 28 supplies current tothe shim coil 22 so that the static magnetic field becomes uniform.

The gradient coil 23 includes an X-axis gradient coil 23 x, a Y-axisgradient coil 23 y and a Z-axis gradient coil 23 z. Each of the X-axisgradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z which is cylinder-shaped is arranged inside thestatic field magnet 21. The gradient coil 23 has also a bed 37 in thearea formed inside it which is an imaging area. The bed 37 supports anobject P. Around the bed 37 or the object P, the RF coil 24 may bearranged instead of being built in the gantry.

The gradient coil 23 communicates with the gradient power supply 27. TheX-axis gradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z of the gradient coil 23 communicate with the X-axisgradient power supply 27 x, the Y-axis gradient power supply 27 y andthe Z-axis gradient power supply 27 z of the gradient power supply 27,respectively.

The X-axis gradient power supply 27 x, the Y-axis gradient power supply27 y and the Z-axis gradient power supply 27 z supply currents to theX-axis gradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z respectively so as to generate gradient magneticfields Gx, Gy and Gz in the X, Y and Z directions in the imaging area.

The RF coil 24 communicates with the transmitter 29 and the receiver 30.The RF coil 24 has a function to transmit an RF signal given from thetransmitter 29 to the object P and receive a MR signal generated due toa nuclear spin inside the object P which is excited by the RF signal togive to the receiver 30.

The sequence controller 31 of the control system 25 communicates withthe gradient power supply 27, the transmitter 29 and the receiver 30.The sequence controller 31 has a function to store sequence informationdescribing control information needed in order to make the gradientpower supply 27, the transmitter 29 and the receiver 30 drive andgenerate gradient magnetic fields Gx, Gy and Gz in the X, Y and Zdirections and an RF signal by driving the gradient power supply 27, thetransmitter 29 and the receiver 3Q according to a predetermined sequencestored. The control information above-described includes motion controlinformation, such as intensity, impression period and impression timingof the pulse electric current which should be impressed to the gradientpower supply 27.

The sequence controller 31 is also configured to give raw data to thecomputer 32. The raw data is complex data obtained through the detectionof an NMR signal and A/D conversion to the NMR signal detected in thereceiver 30.

The transmitter 29 has a function to give an RF signal to the RF coil 24in accordance with control information provided from the sequencecontroller 31. The receiver 30 has a function to generate raw data whichis digitized complex number data by detecting an NMR signal given fromthe RF coil 24 and performing predetermined signal processing and A/Dconverting to the NMR signal detected. The receiver 30 also has afunction to give the generated raw data to the sequence controller 31.

In addition, a breath detection unit 38 for detecting a breath signalfrom the object P is provided with the magnetic resonance imagingapparatus 20. The breath detection signal acquired by the breathdetection unit 38 is outputted to the computer 32 through the sequencecontroller 31.

The computer 32 gets various functions by the operation unit 35executing some programs stored in the storage unit 36 of the computer32. Alternatively, some specific circuits having various functions maybe provided with the magnetic resonance imaging apparatus 20 instead ofusing some of the programs.

FIG. 2 is a functional block diagram of the computer 32 shown in FIG. 1.

The computer 32 functions as an imaging condition setting unit 40, asequence controller control unit 41, a K-space database 42, an imagereconstruction unit 43, an image database 44, an image processing unit45 and a data correction unit 46 by program. The imaging conditionsetting unit 40 includes an acoustic control sequence setting unit 40A,a correction use sequence setting unit 40B, an EPI (echo planar imaging)sequence setting unit 40C and a dummy shot time setting unit 40D.

The imaging condition setting unit 40 has a function to set imagingconditions including a pulse sequence based on instruction informationfrom the input device 33 and to provide the set imaging conditions tothe sequence controller control unit 41. Especially, the imagingcondition setting unit 40 can set an acoustic control sequence forcontrolling a sound generated due to a gradient magnetic field, and acorrection use sequence for acquiring data for correcting a datadeviation due to respiratory body motion of the object P, as well as animaging sequence for acquiring data for imaging and a dummy shotsequence precedently to an imaging sequence. Further, imaging conditionscan be set so as to acquire data under breath synchronization with abreath detection signal obtained by the breath detection unit 38.

The acoustic control sequence setting unit 40A has the function to setan acoustic control sequence. The correction use sequence setting unit40B has the function to set a correction use sequence. The EPI sequencesetting unit 40C has the function to set an EPI sequence as an acousticcontrol sequence, an imaging sequence and/or a correction use sequence.

The dummy shot time setting unit 40D has the function to set anexecution time of a dummy shot sequence. Specifically, the dummy shottime setting unit 40D has the function to set a dummy shot sequencewhich does not acquire data for imaging between plural imaging sequencesto acquire continuously data for imaging in synchronization with abreath detection signal representing a breathing level due torespiratory body motion obtained by the breath detection unit 38, and toelongate and contract the execution time of the dummy shot sequence.

In a dummy shot sequence, a dummy shot by which at least a gradientmagnetic field for slice selection is applied and the same RF pulse asthat in an imaging sequence is transmitted without disturbing a TR isperformed. In a dummy shot sequence, data acquisition may not beperformed by applying no gradient magnetic field for readout, oralternatively, acquired data may not be used for imaging though dataacquisition is performed while applying a gradient magnetic field forreadout.

An acoustic control sequence can be set between dummy shot sequences. Ina dummy shot sequence, a gradient magnetic field pulse can be appliedarbitrarily with regard to a phase encode axis and another frequencyaxis when an appropriate slice is selected by the application of agradient magnetic field pulse for slice selection. In this case,gradient magnetic fields can be set in the positive direction and thenegative direction so that the time integration of a gradient magneticfield per TR becomes zero by canceling each other. For this reason,applying gradient magnetic field pulses having various pulse waveformsin various directions and generating gradient magnetic fields cangenerate a desired sound. That is, adding a gradient magnetic fieldpulse to a dummy shot sequence can produce an acoustic control sequence.Meanwhile, the sound according to a gradient magnetic field determinedfor data acquisition is generated in an imaging sequence.

FIG. 3 is a diagram showing timing for performing an acoustic controlsequence set by the imaging condition setting unit 40 shown in FIG. 2 insynchronization with breathing.

In FIG. 3 the abscissa axis denotes time. As shown in FIG. 3, a breathsignal that changes periodically can be obtained from the breathdetection unit 38. The obtained breath signal is supplied to thecomputer 32. Then in the acoustic control sequence setting unit 40A,trigger signals (TRIGGER1, TRIGGER2, TRIGGER3, . . . ) can beperiodically generated at timings where a level of the breath signalreaches a predetermined reference repeatedly. Moreover, in order to keepthe time for breath to stabilize before the start of an imagingsequence, a period from completion of the last imaging sequence topassage of a predetermined delay time (DELAY TIME 1, DELAY TIME 2, DELAYTIME 3, . . . ) after the trigger signal can be a dummy shot sequencefor acquiring no data for imaging.

An imaging sequence to acquire data for imaging starts after a dummyshot sequence, and the imaging sequence is set so as to finish earlierthan a breathing period which is the interval between trigger signals inconsideration of fluctuation of the breath. That is, the dummy shot timesetting unit 40D is configured to be able to recognize a roughness ofbreath by detecting a period of a trigger signal, and adjust atermination time of an imaging sequence as well as a delay time. Thiscan keep a TR of an imaging sequence constant. The dummy shot timesetting unit 40D can control to discard the acquired one period datathat starts from a trigger signal when receiving the trigger signalduring execution of an imaging sequence.

As mentioned above, a dummy shot sequence for acquiring no data forimaging and an imaging sequence for acquiring data for imaging are setperiodically and repeatedly. Further, adding a gradient magnetic fieldpulse to a dummy shot sequence can create an acoustic control sequence.For this reason, an acoustic control sequence and an imaging sequenceare performed repeatedly in synchronization with a breath signalgenerated due to respiratory body motion.

An acoustic control sequence can be created by setting a gradientmagnetic field pulse for generating a sound. This means varyingintensity and/or a frequency of a gradient magnetic field pulse changesan intensity and a frequency of a sound generated due to application ofthe gradient magnetic field pulse. Consequently, adjusting intensityand/or a frequency of a gradient magnetic field pulse can control asound.

FIG. 4 is a diagram showing an example of acoustic control sequenceshown in FIG. 3.

In FIG. 4 the abscissa axis denotes time. A sound differs between agradient magnetic field pulse Gh such as an EPI sequence that changes ata high frequency as shown in FIG. 4( a) and a gradient magnetic fieldpulse G1 that changes at a low frequency as shown in FIG. 4( b).Consequently, for example, a sound can be changed by varying a frequencyof a readout gradient magnetic field pulse waveform.

FIG. 5 is a diagram showing another example of acoustic control sequenceshown in FIG. 3.

In FIG. 5, the abscissa axis denotes time. As shown in FIG. 5, changingan intensity G of a gradient magnetic field pulse gradually andadjusting its envelope shape can generate a sound having a lowfrequency.

The acoustic control like this can be performed according to a breathsignal. For example, at least one history of an interval of triggersignals is observed. Practically, the most recent three or fourhistories of the interval of trigger signals are observed. When it isdetermined that the interval of triggers became longer, the breath isconsidered to be sluggish and longer. Then, to change one or both of afrequency of a sound and a sound pressure so as to increase encouragesthe object to accelerate breathing. This means a sound and/or a soundpressure can be determined according to a time variation of the pasttrigger intervals obtained from a breath signal.

Note that to suppress the generated sound by application of a gradientmagnetic field pulse whose intensity is zero is also included in anacoustic control. Further, a tone of sound can be controlled byadjusting an application direction of a gradient magnetic field pulsesince respective fixed intensities of gradient magnetic field coils inthe x, y and z directions mutually differ.

In the meantime, a level of a breath signal obtained from the object Palso changes according to a breathing condition (depth). Therefore, abreath signal is automatically adjusted with a gain to acquire a triggersignal in sufficient accuracy into a breath signal having a certainlevel. For this reason, a feedback according to a depth of a breath aswell as a breath interval can be performed by controlling a soundaccording to not only a period of a breath signal but also a changehistory of a gain used for adjustment of the breath signal. In thiscase, the acoustic control sequence setting unit 40A is configured toobtain a gain from the breath detection unit 38 and set the acousticcontrol sequence according to the obtained gain.

Performing the above-mentioned acoustic control sequence to control asound can stabilize a breathing level and a breathing period of theobject P further to reduce an image artifact caused by a respiratorybody motion. In addition, since the stabilization of breath reducesdisturbance of phase and/or amplitude in acquired data, more ideal andfavorable correction can be performed in case of performing theafter-mentioned correction of body motion.

Note that an arbitrary sequence can be used as an acoustic controlsequence and an imaging sequence. For example, a satisfactory effect canbe obtained by using a sequence such as a multi-slice sequence under SE(spin echo) method affected strongly by a change of a TR, i.e., of whichTR is not over 1000 for acquiring longitudinal relaxation (T1) weightedimage data, or a sequential multi-slice sequence under FFE (fast fieldecho) method whose TR is not over 20 for acquiring pieces of image dataone by one.

On the other hand, with an acoustic control sequence or in place of anacoustic control sequence, a correction use sequence can be set betweenimaging sequences in synchronization with a breath signal. A correctionuse sequence is a sequence to acquire data for correcting a datadeviation due to respiratory body motion of the object P as describedabove. An FFE sequence and an EPI sequence can be used as a correctionuse sequence, for example. If a TR of an imaging sequence is long, anEPI sequence gives more satisfactory effect than an FFE sequence.

FIG. 6 is a diagram showing timing for performing a correction usesequence set by the imaging condition setting unit 40 shown in FIG. 2 insynchronization with breathing.

In FIG. 6 the abscissa axis denotes time. Similarly to the acousticcontrol sequence shown in FIG. 3, a correction use sequence can be setbetween imaging sequences in synchronization with a breath signal asshown in FIG. 6. Then, the movement correction of data acquired by acertain imaging sequence can be performed based on data for correctionacquired by a correction use sequence before or after the imagingsequence.

FIG. 7 is a diagram explaining relationship between correction use dataacquired by the correction use sequence shown in FIG. 6 and arespiratory motion amount of the object P.

FIG. 7 shows a abdominal cross section of the object P. In FIG. 7, theabscissa axis is set as the X-axis and the ordinate axis is set as theY-axis. An abdomen 60 varies in the Y-axis direction according to abreathing period between the abdomen 61 during inspiration and theabdomen 62 during expiration. For this reason, an amount of movement inthe Y-axis direction due to a respiratory body motion needs to beobtained and the movement correction of data needs to be performedaccording to the amount of movement. Therefore, information regarding arespiratory body motion needs to be acquired as data for correction.

For example, a correction use sequence to acquire one-dimensional (1D)data for correction by applying a readout gradient magnetic field in theY-axis corresponding to the anterior-posterior direction of the object Pcan be created.

FIG. 8 is a diagram showing an example case of setting one dimensionalsequence as the correction use sequence shown in FIG. 6.

In FIG. 8, the abscissa axis denotes time, Gz, Gx and Gy denote gradientmagnetic fields in Z axis, X axis and Y axis directions, respectively,and ADC denotes correction use data acquired as an echo signal.

As shown in FIG. 8, by setting a correction use sequence for applying agradient magnetic field pulse Gss for slice selection in the Z-axisdirection and a gradient magnetic field pulse Gro for readout in theY-axis direction, correction use data ADC can be obtained withoutapplying a gradient magnetic field pulse for phase encode. An echosignal obtained by applying a gradient magnetic field pulse in a readoutdirection and setting a phase encode amount to be zero is sometimescalled a navigator echo. This one-dimensional correction use dataacquired as K-space (Fourier space) data shifts in phase by an amountdepending on the amount of movement in the Y-axis direction due tobreath of the object P. Therefore, by regarding the phase shift amountof the correction use data as phase correction data, to correct a phaseof K-space data acquired by performing an imaging sequence so that theamount of movement in the Y-axis direction is cancelled can performmovement correction.

However, although in most cases an amount of movement in the Z-axisdirection corresponding to the body axis direction of the object P canbe calculated with a practical accuracy by acquiring one-dimensionalcorrection use data, an amount of movement in the Y-axis directioncorresponding to the anterior-posterior direction of the object P mightnot be obtained with a practical accuracy by acquiring one-dimensionalcorrection use data. Therefore, a correction use sequence for acquiringtwo-dimensional image data with a low resolution in the X-axis and theY-axis directions as correction use data can be created.

Performing two-dimensional acquisition and using image data as thecorrection use data can calculate a correction amount for correctiondepending on a positional shift amount in each direction (especially inthe Y-axis direction).

FIG. 9 is a diagram showing an example case of setting two-dimensionalsequence as the correction use sequence shown in FIG. 6.

In FIG. 9, the abscissa axis denotes time, Gz, Gx and Gy denote gradientmagnetic fields in Z axis, X axis and Y axis directions, respectively,and ADC denotes correction use data acquired as an echo signal.

As shown in FIG. 9, a correction use sequence to obtain two-dimensionalimage data with a low resolution in the X-axis and the Y-axis directioncan be created by an EPI sequence applying a gradient magnetic fieldpulse for frequency encode whose frequency is higher than that of agradient magnetic field pulse for frequency encode of an imagingsequence, for example. In this case, an amount of movement of the objectP in the Y-axis direction can be obtained from two-dimensional imagedata as an amount or positional shift, and the positional correction ofimage data acquired by imaging can be performed by using the obtainedamount of positional shift. Further, correction use image data can betemporarily converted into K-space data and a phase of K-space data canbe corrected by a phase shift, corresponding to the positional shift, inK-space.

Note that, the movement of the object P can be considered as not only alinear motion of a rigid body accompanied by a translational motionincluding only a parallel shift and a rotation but also a nonlinearmovement of a non-rigid body including scaling (expansion andcontraction movement) and shear deformation. When a movement isnonlinear, a correction method, by which K-space is divided according toa size of nonlinear motion measured or predicted based on correction usedata in the real space and mutual weighting addition of respectivepieces of data corrected by performing different phase correction toeach divided K-space is performed, can be employed.

Meanwhile, not only a correction use sequence as described above, butalso an acoustic control sequence can be set between imaging sequences.In this case, since a gradient magnetic field pulse shape is limited inorder to acquire correction use data, a gradient magnetic field pulsefor acoustic control must be set to an extent in which the correctionuse data can be acquired. For example, when correction use data isacquired under an EPI sequence, the acoustic control can also beperformed with acquiring the correction use data by setting a gradientmagnetic field pulse such as an end spoiler unnecessary for acquiringthe correction use data to the EPI sequence.

Then, other functions of the computer 32 will be described.

The sequence controller control unit 41 has a function for controllingthe driving of the sequence controller 31 by giving imaging conditionsincluding a pulse sequence, acquired from the imaging condition settingunit 40, to the sequence controller 31 in response to informationinstructing scan start from the input device 33. Further, the sequencecontroller control unit 41 has a function for receiving raw data fromthe sequence controller 31 and arranging the raw data to K-space formedin the K-space database 42. Therefore, the K-space database 42 storesthe raw data generated by the receiver 30 as K-space data. That is,K-space data is arranged in K-space formed in the K-space database 42.

The image reconstruction unit 43 has a function for reconstructing imagedata, which is real space data, of the object P from K-space data bycapturing K-space data from the K-space database 42 and performing imagereconstruction processing including FT (Fourier Transformation) toK-space data, and writing the obtained image data to the image database44. Therefore, the image database 44 stores the image data reconstructedby the image reconstruction unit 43.

The image processing unit 45 has a function for generatingtwo-dimensional image data for displaying by performing necessary imageprocessing to image data read form the image database 44 and displayingthe generated image data on the display unit 34.

The data correction unfit 46 has the function to read K-space data orimage data acquired as correction use data from the K-space database 42or the image database 44, then perform a movement correction to K-spacedata or image data for imaging in an arbitrary method by usingcorrection use data, and to write K-space data or image data after themovement correction in the K-space database 42 or the image database 44.An average (zero-dimensional) phase shift in the real space becomesone-dimensional phase shift in K-space. Further, the positional shift inthe real space becomes one-dimensional phase shift in K-space.

Therefore, for example, when one-dimensional or two-dimensional K-spacedata for correction is acquired by performing a correction use sequence,the phase of K-space data acquired by performance of an imaging sequencecan be corrected in the data correction unit 46 with the use of K-spacedata for correction read from the K-space database 42. Morespecifically, a phase error Δφ in each shot can be calculated fromK-space data for correction acquired by a reference shot n0 and fromK-space data for correction acquired by another shot n as shown inexpression (1).

φ=2πYKy

φ0=2πY0Ky

Δφ=φ−φ0  (1)

wherein Y denotes a shift amount due to motion for a shot n and Y0denotes a shift amount due to motion for reference shot n0.

When the phase error Δφ in each shot is calculated, phase correction canbe performed by multiplying exp(−Δφ) by data for imaging correspondingto each shot.

Meanwhile, when two-dimensional image data for correction is acquired byperforming the correction use sequence, in the data correction unit 46,positions of image data acquired by performing the imaging sequence canbe also corrected by using correction use image data read from the imagedatabase 44. That is, positional shift distances ΔY in the respectiveshots can be obtained based on image data for correction acquired by areference shot and from image data for correction acquired by anothershot. Then, positions can be corrected by subtracting the positionalshift distances ΔY corresponding to the respective shots from pieces ofimage data for imaging corresponding to the respective shotsrespectively. Alternatively, phases can also be corrected by multiplyingeach pieces of K-space data for imaging by exp(−2πKyΔY).

In addition, a nonlinear motion correction can be performed by dividingK-space and obtaining a phase error corresponding to each dividedK-space to perform different phase correction as mentioned above.

Then, the operation and action of a magnetic resonance imaging apparatus20 will be described.

FIG. 10 is a flowchart for generating an image of the object P whileacoustically controlling and applying motion correction regardingrespiratory body motion synchronized with breathing using the magneticresonance imaging apparatus 20 shown in FIG. 1.

First, in step S1, the imaging condition is set where an acousticcontrol sequence and a correction use sequence are performed betweenimaging sequences performed repeatedly with breath synchronization bythe imaging condition setting unit 40. This means the dummy shot timesetting unit 40D sets a period of a dummy shot sequence between imagingsequences and the correction use sequence setting unit 40B sets acorrection use sequence for acquiring correction use data in the periodof the dummy shot sequence as shown in FIG. 8 or FIG. 9. Further, theacoustic control sequence setting unit 40A makes the correction usesequence into an acoustic control sequence by adding a gradient magneticfield pulse for an acoustic control to the correction use sequence. Inthis instance, when any of the correction use sequence, the acousticcontrol sequence and the imaging sequence is an EPI sequence, thecorresponding EPI sequence is created by the EPI sequence setting unit40C.

Then in step S2, an imaging scan is performed in synchronization with abreath signal. This means operation instruction is provided from theinput unit 33 to the sequence controller control unit 41. Then, an RFsignal is transmitted to the object P, and raw data is acquired from theobject P.

For this purpose, the object P is set to the bed 37 in advance, and astatic magnetic field is generated at an imaging area of the magnet 21(a superconducting magnet) for static magnetic field excited by thestatic-magnetic-field power supply 26. Further, the shim-coil powersupply 28 supplies current to the shim coil 22, thereby uniformizing thestatic magnetic field generated at the imaging area.

Then, the input device 33 sends instruction for starting a scan to thesequence controller control unit 41. The sequence controller controlunit 41 supplies the imaging conditions including the acoustic controlsequence, the correction use sequence and the imaging sequence receivedfrom the imaging condition setting unit 40 to the sequence controller31. Therefore, the sequence controller 31 drives the gradient powersupply 27, the transmitter 29, and the receiver 30 in accordance withthe imaging conditions received from the sequence controller controlunit 41, thereby generating gradient magnetic fields in the imaging areahaving the set object P, and further generating RF signals from the RFcoil 24.

Consequently, the RF coil 24 receives NMR signals generated due tonuclear magnetic resonance in the object P. Then, the receiver 30receives the NMR signals from the RF coil 24 and generates raw datawhich is digital data of NMR signals by A/D conversion subsequently tonecessary signal processing. The receiver 30 supplies the generated rawdata to the sequence controller 31. The sequence controller 31 suppliesthe raw data to the sequence controller control unit 41. The sequencecontroller control unit 41 arranges the raw data as K-space data toK-space formed in the K-space database 42.

In such a scan, sound is controlled by performing the acoustic controlsequence. For example, the sound changes are approximately synchronizedwith an interval of breaths. Therefore, breaths are stabilized, anddisplacements of a breathing period and a breath level of the object Pare reduced. As a result, disorders of phase and amplitude in acquiredK-space data are reduced.

Then in step S3, movement correction of data based on the correction usedata and image reconstruction are performed.

When K-space data is employed as the correction use data, the datacorrection unit 46 calculates a phase difference corresponding to anamount of movement in the anterior-posterior direction of the object Pas a correction coefficient based on the correction use data stored inthe K-space database 42. Then, phases of K-space data for imagingacquired by performing the imaging sequence are corrected by thecorrection coefficients so that the obtained phase difference iscancelled. The phase-corrected K-space data is written in the K-spacedatabase 42 again. Subsequently, the image reconstruction unit 43reconstructs image data by retrieving K-space data after the correctionfrom the K-space database 42 and performing image reconstructionprocessing. The image data obtained by reconstitution is written in theimage database 44.

Meanwhile, when two-dimensional image data with a low resolution is usedas correction use data, the image reconstruction unit 43 reconstructspieces of image data for display and correction by retrieving K-spacedata before the correction and K-space data acquired as the correctionuse data from the K-space database 42 and performing imagereconstruction processing. The pieces of image data for display andcorrection obtained by reconstitution are written in the image database44. Then, the data correction unit 46 calculates a positionaldisplacement amount corresponding to an amount of movement in theanterior-posterior direction of the object P as a correction coefficientbased on the correction use image data stored in the image database 44.Then, positions of the image data for display are corrected by thecorrection coefficients so that the obtained positional displacementamount is cancelled. The image data after the correction is written inthe image database 44.

Note that, when a shift due to respiratory body motion is large to makeK-space data inappropriate to use for generating image data, it can alsobe avoided to use a specific piece of K-space data for imagereconstruction by removal processing of error data in the datacorrection unit 46.

Subsequently, in step S4, two-dimensional image data for display isgenerated from the image data after the movement correction anddisplayed on the display unit 34. Specifically, the image processingunit 45 retrieves the image data after the movement correction from theimage database 44 and generates the two-dimensional image data fordisplay by necessary image processing. Then, the generated image datafor display is displayed on the display unit 34.

In this instance, since the image data displayed on the display unit 34is generated, accompanied by the respiratory movement correction, basedon the data acquired under breathing stabilized by the acoustic control,it becomes image data with a reduced artifact caused by respiratory bodymotion. Therefore, a user can make a diagnosis based on the diagnosticimage with a reduced artifact.

That is, the magnetic resonance imaging apparatus 20 as mentioned aboveis designed to be able to perform stabilization of breath of the objectP and/or movement correction of acquired data by performing an acousticcontrol sequence and/or a correction use sequence precedently to animaging sequence. Note that, an acoustic control sequence and/or acorrection use sequence can be also performed in imaging without breathsynchronization. However, improving the stabilization of breath can beexpected by performing an acoustic control according to a breath signal.Further, correction use data and data for imaging can be acquired at anappropriate timing and at an appropriate breath signal level byacquiring correction data in synchronization with breath.

For this reason, the magnetic resonance imaging apparatus 20 can performa longer imaging than breath-holding imaging. Since a breath level and abreathing period become constant by the stabilization of breath of theobject P, a higher SNR image can be obtained with a higher resolution bythe movement correction especially in case of T1 weighted imaging.

Note that, another artifact reduction technique can be used at the sametime. The technique called HIGH SORT or LOW SORT can be cited as anexample of an available combination technique of the artifact reduction.In conjunction with these techniques, effect of decreasing of anartifact caused by respiratory body motion can be improved and ahigh-SNR image can be obtained with a high resolution.

FIG. 11 is a diagram explaining a technique for reducing artifactreferred to as HIGH SORT which can be applied to the magnetic resonanceimaging apparatus 20 shown in FIG. 1.

As shown in FIG. 11, HIGH SORT is a correction technique for reducing anartifact by sorting the order of phase encoding depending on a breathingperiod to assign the breathing period to the maximum frequency inK-space. When HIGH SORT is performed, a sequence according to the orderof data acquisition is produced in the imaging condition setting unit40. Then, data sorting is performed in K-space.

FIG. 12 is a diagram explaining a technique for reducing artifactreferred to as LOW SORT which can be applied to the magnetic resonanceimaging apparatus 20 shown in FIG. 1.

As shown in FIG. 12, LOW SORT is a correction technique for reducing anartifact by sorting the order of phase encoding depending on a breathingperiod to assign the breathing period to the minimum frequency inK-space. When LOW SORT is performed, a sequence according to the orderof data acquisition is also produced in the imaging condition settingunit 40. Then, data sorting is performed in K-space.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A magnetic resonance imaging apparatuscomprising: a correction data acquisition unit configured to acquirecorrection use data for correcting an influence of a respiratory bodymotion on image data in synchronization with a signal representing therespiratory body motion, the correction use data being two-dimensionalimage data in an X-axis and a Y-axis direction, and the correction usedata being obtained by applying both readout gradient and phase encodegradient; a data acquisition unit configured to acquire imaging data byimaging subsequently to acquiring the correction use data; and acorrection unit configured to generate image data by correcting theinfluence of the respiratory body motion based on a position shift ineach of the X-axis and Y-axis directions in the correction use data. 2.A magnetic resonance imaging apparatus comprising: a correction dataacquisition unit configured to acquire correction use data forcorrecting an influence of a respiratory body motion on image data, thecorrection use data being two-dimensional image data in an X-axis and aY-axis direction, and the correction use data being obtained by applyingboth readout gradient and phase encode gradient; an acoustic controlunit configured to control a sound by applying a gradient magnetic fieldwith a purpose other than acquiring the correction use data; a dataacquisition unit configured to acquire imaging data by imagingsubsequently to acquiring the correction use data and controlling thesound; and a correction unit configured to generate image data bycorrecting the influence of the respiratory body motion based on aposition shift in each of the X-axis and Y-axis directions in thecorrection use data.
 3. The magnetic resonance imaging apparatus ofclaim 1, wherein said correction data acquisition unit is configured toacquire the correction use data according to an echo planar imagingsequence.
 4. The magnetic resonance imaging apparatus of claim 1,wherein said correction data acquisition unit is configured to acquirethe correction use data according to a fast field echo sequence.
 5. Themagnetic resonance imaging apparatus of claim 1, wherein said correctiondata acquisition unit is configured to acquire the correction use datawhile applying a gradient magnetic field for frequency encode having afrequency higher than a frequency of a gradient magnetic field forfrequency encode applied for acquiring the imaging data.
 6. The magneticresonance imaging apparatus of claim 1, wherein said correction dataacquisition unit is configured to acquire the correction use data aseither of (a) information regarding the respiratory body motion and (b)information regarding a phase of the imaging data.
 7. The magneticresonance imaging apparatus of claim 1, further comprising an adjustingunit configured to adjust a period for acquiring the correction use datain accordance with the signal representing the respiratory body motionwhile keeping the repetition time for acquiring the imaging dataconstant.
 8. A magnetic resonance imaging method comprising: acquiringcorrection use data for correcting an influence of a respiratory bodymotion on image data in synchronization with a signal representing therespiratory body motion, the correction use data being two-dimensionalimage data in an X-axis and a Y-axis direction, and the correction usedata being obtained by applying both readout gradient and phase encodegradient; acquiring imaging data by imaging subsequently to acquiringthe correction use data; and generating image data by correcting theinfluence of the respiratory body motion based on a position shift ineach of the X-axis and Y-axis directions in the correction use data. 9.The magnetic resonance imaging apparatus of claim 1, wherein thecorrection data acquisition unit is configured to acquire the correctionuse data while applying a gradient magnetic field for frequency encodehaving a frequency higher than a frequency of a gradient magnetic fieldfor frequency encode applied for acquiring the imaging data.
 10. Themagnetic resonance imaging apparatus of claim 1, wherein the correctionunit performs positional correction of the image data by using theposition shift in the correction use data.
 11. The magnetic resonanceimaging apparatus of claim 1, wherein the correction unit corrects theimaging data in K-space acquired by the data acquisition unit by using aphase shift that is converted from the position shift in the correctionuse data.
 12. The magnetic resonance imaging apparatus comprising: acorrection data acquisition unit configured to acquire correction usedata for correcting an influence of a respiratory body motion on imagedata in synchronization with a signal representing the respiratory bodymotion, the correction use data being two-dimensional image data in anX-axis and a Y-axis direction, and the correction use data beingobtained by applying both readout gradient and phase encode gradient;and a data acquisition unit configured to acquire imaging data byimaging subsequently to acquiring the correction use data.
 13. Themagnetic resonance imaging apparatus of claim 12, wherein the dataacquisition unit sorts an order of phase encoding of the imaging data inK-space based on the correction use data.